1. Field of the Invention
The present invention relates to a nickel hydrogen storage battery.
2. Description of the Related Art
A nickel hydrogen storage battery is used for various purposes. Because of its high output power, the nickel hydrogen storage battery is also applied to a power source for a vehicle, such as a hybrid vehicle and an electric vehicle. When used for such a vehicle, the nickel hydrogen storage battery is required to exert its high output power and capacity retention characteristics. A conventional way of achieving high output is, for example, increasing the facing area of positive and negative electrode plates while making the plates and a separator thin. Examples of a measure for improving the capacity retention characteristics include the trapping of nitric-acid-group shuttle by applying a sulfonated separator and the prevention of self-decomposition, i.e., discharge reaction caused exclusively in a positive electrode due to oxygen generation by adding rare-earth elements to the positive electrode.
If the separator is designed to be thinner for increasing the facing area, (1) it is required to increase the density of a nonwoven fabric composing the separator to prevent a short circuit; and (2) in light of electrolyte retention of the separator, the fibers of the nonwoven fabric need to be finer to increase the surface area of the fibers. The nonwoven fabric of a battery separator which is disclosed in Unexamined Japanese Patent Publication No. 2004-335159 is composed of microfibers of 40 percent by mass or less (a fiber diameter of less than 4 μm) and compound high-strength polypropylene-based fibers including a melting component of 60 percent by mass or more. This nonwoven fabric can be considered to meet the above conditions (1) and (2) to some extent.
The sulfonated separator is prone to be degraded in strength as the fibers composing the separator get damaged, as compared to a separator subjected to another hydrophilizing treatment such as fluorine gas treatment. For this reason, a cylindrical nickel hydrogen storage battery using a sulfonated separator has the possibility of an internal short circuit caused by positive and negative electrode plates breaking the separator and coming into direct contact to each other.
The sulfonated separator is inferior in liquid absorbability to separators subjected to other hydrophilizing treatments than sulfonation treatment. Especially in a high output battery, it is required to secure a transmission path, i.e., conductive path for ion in between positive and negative electrode plates, so that an electrolytic solution is permeated into the separator by vacuum injection. In this respect, a separator subjected to the fluorine gas treatment is excellent in liquid absorbability and hydrophilicity. In a nickel hydrogen storage battery using a separator subjected to fluorine gas treatment, the electrolytic solution continues to be permeated into the separator because of the liquid absorbability of the separator itself even after injection. As a result, the conductive path is secured in full measure, which provides high output. Plasma treatment, surface-active agent (surfactant) treatment and the like also produce the same effects as the fluorine gas treatment.
A cylindrical nickel hydrogen storage battery disclosed in Unexamined Japanese Patent Publication No. 2004-31293 uses two kinds of separators in connection with relationship of self-discharge prevention with short-circuit resistance and output characteristics.
Among the two separators of the battery described in Publication No. 2004-31293, one of them is subjected, for example, to fluorine gas treatment to keep strength, and the other to sulfonation treatment. The fluorine gas-treated separator is disposed outside of a positive electrode plate, and the sulfonated separator inside of the positive electrode plate.
However, a nickel hydrogen storage battery in which a sulfonated battery separator described in Unexamined Japanese Patent Publication No. 2004-335159 is used, and in which a negative electrode plate includes a nonaqueous polymer binder for preventing an increase of internal pressure of the battery, has the problem that output is inversely decreased if the area of polar plates are increased to a certain value or more. This problem stands out especially when a sulfonated separator like the one disclosed in Publication No. 2004-31293 and a separator subjected to the fluorine gas treatment are used at the same time.